Operations Overview of Existing Mass Spectrometer Using Plasma Ion Source
Known as one example of a mass spectrometer using a plasma ion source is an inductively coupled plasma mass spectrometer (ICP-MS) using inductively coupled plasma (ICP) as an ion source for ionizing an element in a measurement sample. Operations of such a known inductively coupled plasma mass spectrometer are summarized with reference to FIG. 7, which is a block diagram thereof. In FIG. 7, an optional autosampler 10 or a sample suction tube connected by an operator to a sample introduction unit 15 is wetted in a measurement sample 5 in a sample bottle and the sample 5 is introduced from the sample introduction unit 15 into an ionization unit 20 such that an element included in the sample 5 is ionized by plasma generated in the ionization unit 20. The ionized element is sampled at an interface unit 25 configuring a differential exhaust system including a sampling cone and a skimmer cone; introduced into a high-vacuum chamber having an ion-lens unit 30, a mass separator 35, and a detector 42 therein; converged by the ion-lens unit 30; and afterward injected into the mass separator 35, which is for transmitting only ions of a selected mass-to-charge ratio and is typically configured from a quadrupole mass filter.
The detector 42 is typically configured from a secondary-electron multiplier and outputs an electrical signal corresponding to a number of ions of the mass-to-charge ratio separated by the mass separator 35 that reaches the detector 42 per unit time. The electrical signal output from the secondary-electron multiplier is sent to a pulse counter 44 and an analog current measurement unit 46, and a pulse-count value according to a pulse frequency of the electrical signal and an analog current value of the electrical signal are respectively measured by the pulse counter 44 and the analog current measurement unit 46. The detector 42, the pulse counter 44, and the analog current measurement unit 46 configure an ion measurement unit 40.
An ion-lens voltage drive unit 55 operates so as to apply a voltage to an ion lens in the ion-lens unit 30. The ion lens is configured from an electrostatic-lens group having an action of changing a trajectory of an ion using an electrical field and is configured such that when a voltage applied to an electrode thereof changes, an ion transmission rate changes accordingly. Because of this, by controlling the ion-lens voltage drive unit 55 by a system control unit 60 in order to change the voltage applied to the electrode of the ion lens as appropriate, the ion transmission rate of the ion lens can be increased or decreased. At a time of normal measurement, the voltage applied to the ion lens is set to a predetermined voltage so a transmission rate of an ion of an isotope of an analysis element whose ionic strength is to be measured is maximized in order to determine a concentration of the analysis element in the measurement sample.
The system control unit 60 controls operations of each block in FIG. 7, and a computational processing unit 65 performs data processing such as converting the measured analog current value into an ion count per second (cps) for each mass-to-charge ratio (m/z). Note that it is also possible to connect the mass spectrometer and an external computing device 70 such as a PC (personal computer) via a network or the like to transfer data such as a measurement value of ionic strength (ion count) to the computing device 70 and perform computational processing seeking the ionic strength of the ion of the isotope of the analysis element to be measured and input/output processing with a user.
By making polarities identical for two opposing rod electrodes among four parallel rod electrodes configuring a quadrupole mass filter of the mass separator 35 (a polarity of one pair of opposing rod electrode being opposite a polarity of the other pair of rod electrodes) to apply a voltage where a DC voltage and a high-frequency AC voltage are superimposed and setting the voltage of the DC voltage and the voltage of the high-frequency AC voltage as appropriate, only ions of a specified mass-to-charge ratio can be transmitted to reach the detector 42. Moreover, by changing a ratio between the DC voltage and the high-frequency AC voltage applied to these rod electrodes, a mass resolution can be adjusted. Note that this setting of the mass-to-charge ratio and the mass resolution is performed by the system control unit 60 in response to an input setting desired by the operator via the external computing device (70 in FIG. 7) of the mass spectrometer. Moreover, there are mass spectrometers using a plasma ion source that use a sector mass filter, and these devices can adjust the mass resolution by changing a slit width through which the ions pass.
As another example of a mass spectrometer using a plasma ion source, there is a glow-discharge mass spectrometer (GDMS), which uses a glow discharge as a means of ionization.
By measuring the ionic strength of the ion of the isotope of the analysis element by a mass spectrometer using a plasma ion source such as above, the concentration of the analysis element can be determined. Hereinbelow, in the present specification, the ion of the isotope of the analysis element whose ionic strength is to be measured in order to determine this concentration is referred to as a “measurement ion of the analysis element” and the isotope thereof is referred to as a “measurement isotope of the analysis element” (in a situation where a certain specific analysis element is defined as a, these are respectively referred to as a “measurement ion of analysis element α” and a “measurement isotope of analysis element α”).
Conventional Method of Correcting Spectral Interference
When measuring a concentration whereat an analysis element such as arsenic (As) or selenium (Se) is included in a measurement sample such as an environmental or food sample by a mass spectrometer using a plasma ion source such as an ICP-MS (in a situation where the present specification simply refers to a “mass spectrometer,” this signifies a mass spectrometer using a plasma ion source), in a situation where a rare-earth element is included in the sample, a measurement error due to spectral interference may arise in a measurement value of ionic strength of a measurement ion of the analysis element. This spectral interference arises due to a mass-to-charge ratio of the measurement ion of the analysis element and a mass-to-charge ratio of a divalent ion of the rare-earth element in the sample being identical or close such that separation by the mass spectrometer is not possible.
FIG. 1 lists isotope mass numbers (m), isotope abundance ratios, and mass-to-charge ratios of divalent ions (m/2) for each of several rare-earth elements. For example, the mass-to-charge ratio of divalent ion 150Nd2+ of 150Nd (neodymium), a rare-earth element, and the mass-to-charge ratio of divalent ion 150Sm2+ of 150Sm (samarium), which is also a rare-earth element, are both 75, which is identical to the mass number of 75As (although strictly speaking there is a difference, this difference is small, and separation by a mass spectrometer is not possible). Because of this, in a situation where the analysis element in the sample is As and the measurement ion of analysis element As is a 75As ion of a mass-to-charge ratio of 75, if these rare-earth elements are present in the sample, divalent ions thereof cause spectral interference for the 75As ion of the mass-to-charge ratio of 75, preventing the concentration of As in the sample from being accurately determined. Similarly, in a situation where the analysis element in the sample is Se and the measurement ion of analysis element Se is a 78Se ion of a mass-to-charge ratio of 78, divalent ion 156Gd2+ of rare-earth element 156Gd (gadolinium) and divalent ion 156Dy2+ of rare-earth element 156Dy (dysprosium) cause spectral interference for the 78Se ion of the mass-to-charge ratio of 78.
Hereinbelow, in the present specification, elements such as 150Nd and 150Sm above that, when ionized, cause spectral interference for a measurement ion of an analysis element are referred to as interfering elements. Known as a conventional correction method of correcting such spectral interference due to a divalent ion of an interfering element present in a sample is one using a measurement value of ionic strength of a divalent ion of an isotope having an odd mass number among isotopes of the interfering element (non-patent literature 1). This conventional correction method is described below.
An analysis element in a sample is defined as a, and a mass number of a measurement isotope of analysis element α is defined as αn. Here, when analysis element α is ionized, it becomes a monovalent ion. As such, the mass number αn of the measurement isotope of analysis element α and a mass-to-charge ratio of a measurement ion of analysis element α are equal. Therefore, hereinbelow, αn is also used to represent the mass-to-charge ratio of the measurement ion of analysis element α. A certain interfering element present in the sample is defined as X, and respective divalent ions of X1 and X2, two different isotopes of X (respective mass numbers being X1n and X2n), are defined as X12+ and X22+. Here, X2n is odd (a signal of a divalent ion of an isotope of an odd mass number can be accurately measured without interference because a mass-to-charge ratio thereof is not an integer). X12+ causes spectral interference for the measurement ion of analysis element α because a mass-to-charge ratio thereof (X1n/2) is identical to the mass-to-charge ratio αn of the measurement ion of analysis element α or so close to an that separation is not possible at a resolution of the mass spectrometer.
A measurement value of ionic strength at the mass-to-charge ratio αn measured by the mass spectrometer and a measurement value of ionic strength at a mass-to-charge ratio X2n/2 are respectively defined as [αn]m and [X2n/2]m. [X2n/2]m is multiplied by an isotope ratio A1/A2, which is a ratio between a theoretical isotope abundance ratio A1 of X1 and a theoretical isotope abundance ratio A2 of X2, and this subtracted from [αn]m is defined as corrected value [αn]c, where spectral interference due to X12+ is corrected. That is,[αn]c=[αn]m−[X2n/2]m×A1/A2.  [Formula 1-1]
An example is described where, in a situation where As as the analysis element and Nd and Sm as the interfering elements are copresent in the sample, the conventional method above is applied to correct spectral interference due to divalent ions thereof on a 75As ion of a mass-to-charge ratio of 75 that is the measurement ion of the analysis element As. In this situation, as above, 150Nd2+ and 150Sm2+ cause spectral interference for the 75As ion whose mass-to-charge ratio is 75.
First, correction of the spectral interference due to 150Nd2+ is described. Respectively defining a measurement value of ionic strength at a mass-to-charge ratio of 75 as measured by the mass spectrometer and a measurement value of ionic strength at a mass-to-charge ratio of 72.5 as measured by the mass spectrometer (that is, a measurement value of ionic strength of 145Nd2+) as [75]m and [72.5]m, [αn]m and [X2n/2]m in [formula 1-1] respectively correspond to [75]m and [72.5]m. Moreover, an isotope ratio of 150Nd and 145Nd is known to be 150Nd/145Nd=5.6/8.3≈0.675) (see FIG. 1), and this corresponds to A1/A2 in [formula 1-1]. That is, defining an ionic strength at the mass-to-charge ratio of 75 where the spectral interference due to 150Nd2+ is corrected as [75]c, in the present example, [formula 1-1] is expressed as[75]c=[75]m−[72.5]m×5.6/8.3.  [Formula 1-2]
The spectral interference due to 150Sm2+ on the 75As ion of the mass-to-charge ratio of 75 is corrected in a similar manner. That is, by multiplying a measurement value of ionic strength at a mass-to-charge ratio of 73.5 (that is, a measurement value of ionic strength of 147Sm2+) with 150Sm/147Sm, which is the isotope ratio of 150Sm and 147Sm, and subtracting this from the [75]c in [formula 1-2], an ionic strength is obtained where the spectral interference due to both 150Nd2+ and 150Sm2+ on the 75As ion of the mass-to-charge ratio of 75 is corrected.